Influence of random pore defects on failure mode and mechanical properties of SiC ceramics under uniaxial compression using discrete element method

To investigate the relationship between micro-defects in ceramic materials and macro mechanical properties and behaviours, a computational model of SiC ceramics with randomly oriented elliptical pores was established using the discrete element method (DEM). The effects of pore defect content and its aspect ratio on the failure mode, stress-strain curve and mechanical properties of specimen were investigated under uniaxial compression. The effective Young's modulus which was obtained from DEM simulations was compared with the predictions of Mori-Tanaka scheme (MTS) and Self-Consistent scheme (SCS) at various pore defect densities. The results showed that the compressive strength and crack initiation stress decrease nonlinearly as the pore defect content increases. Furthermore, the smaller the aspect ratio of the elliptical pore defects was, the more obvious the weakening trend was. As the pore defect content increases, the failure mode of the specimen changed from brittle fracture to tensile-shear mixing and then to axial splitting. The stress-strain curves showed a certain “softening” period during the loading process. The effective Young's modulus obtained from the DEM simulations coincides with the approximations of MTS and SCS at low pore densities. However, when the pore defect density became larger, the DEM simulation results were slightly lower than the theoretical results of the Mori-Tanaka scheme, which only considers the weak interaction between defects.     

Determination of coalescence kernels for high-shear granulation using DEM simulations

Discrete element method (DEM) modeling is used in parallel with a model for coalescence of deformable surface wet granules. This produces a method capable of predicting both collision rates and coalescence efficiencies for use in derivation of an overall coalescence kernel. These coalescence kernels can then be used in computationally efficient meso-scale models such as population balance equation (PBE) models. A soft-sphere DEM model using periodic boundary conditions and a unique boxing scheme was utilized to simulate particle flow inside a high-shear mixer. Analysis of the simulation results provided collision frequency, aggregation frequency, kinetic energy, coalescence efficiency and compaction rates for the granulation process. This information can be used to bridge the gap in multi-scale modeling of granulation processes between the micro-scale DEM/coalescence modeling approach and a meso-scale PBE modeling approach.     

The effect of matrix morphology on nanocomposite properties

In order to understand the effect of bulk matrix morphology on polymer nanocomposite properties, nanocomposites containing chemically similar but morphologically different polyamide matrices and carbon nanofibers were processed and characterized. Two polyamide matrices were used, one amorphous and one semi-crystalline. Experimental results indicated that the reinforcing efficacy of the amorphous matrix was higher than the semi-crystalline matrix at temperatures below the glass transition. At a carbon nanofiber loading of 0.5 wt.%, the experimentally measured modulus with the amorphous matrix exceeded the Halpin-Tsai prediction for an isotropic material. Overall, these results provided distinct evidence that the underlying bulk matrix morphology plays an important role in polymer nanocomposite mechanical design.     

On the optimal numerical time integration for DEM using Hertzian force models

The numerical accuracy of a selection of different time integration techniques used to solve particle motion is investigated using a normal collision employing the non-linear Hertzian contact force. The findings are compared against the linear force model where it has been found that the expected order of accuracy of higher-order integration schemes is not realised (Tuley et al., 2010). The proposed mechanism for this limitation has been cited as the errors in integration which occur across the force profile discontinuity. By investigating the characteristics of both the non-linear elastic and the non-linear damped Hertzian contact models, it has been found that higher orders of accuracy are recoverable and depends on the degree of the governing non-linear equation. The numerical errors of the linear and non-linear force models are however markedly different in character.     

Application of Taguchi methods to DEM calibration of bonded agglomerates

Discrete element modelling (DEM) is commonly used for particle-scale modelling of granular or particulate materials. Creation of a DEM model requires the specification of a number of micro-structural parameters, including the particle contact stiffness and the interparticle friction. These parameters cannot easily be measured in the laboratory or directly related to measurable, physical material parameters. Therefore, a calibration process is typically used to select the values for use in simulations of physical systems. This paper proposes optimising the DEM calibration process by applying the Taguchi method to analyse the influence of the input parameters on the simulated response of powder agglomerates. The agglomerates were generated in both two and three dimensions by bonding disks and spheres together using parallel bonds. The mechanical response of each agglomerate was measured in a uniaxial compression test simulation where the particle was compressed quasi-statically between stiff, horizontal, frictionless platens. Using appropriate experimental designs revealed the most important parameters to consider for successful calibration of the 2D and 3D models. By analysing the interactive effects, it was also shown that the conventional calibration procedure using a “one at a time” analysis of the parameters is fundamentally erroneous. The predictive ability of this approach was confirmed with further simulations in both 2D and 3D. This demonstrates that a judicious strategy for application of Taguchi principles can provide a sound and effective calibration procedure.     

Impact of functionalised dispersing agents on the mechanical and viscoelastic properties of pigment coating

The effect of poly(acrylic acid) – poly(styrene sulphonic acid) (AA-SSA) and poly(acrylic acid) – poly(2-acrylamido-2-methylpropane sulphonic acid) (AA-AMPS) dispersing agents on the viscoelastic and mechanical properties of precipitated calcium carbonate-latex composites used was investigated. Four different formulations were prepared using carboxylated styrene butadiene (SBR) and styrene-acrylate (SA) latexes. Pore space was characterised using mercury porosimetry. The storage modulus and loss factor were evaluated through dynamic mechanical thermal analysis (DMTA) low frequency single cantilever bending mode. The ultimate tensile strength and the Young modulus were also measured. At low latex contents, storage modulus was found to be higher for SBR latex composites. At high latex content, the SA storage modulus was found to be higher. It is suggested that at lower latex content, the viscoelastic properties are function of physical microstructure, which at low latex is influenced by the latex glass transition temperature (Tg). Softer latex spreads more on the pigment surface providing higher stiffness, since pigments are held together by latex bridges. At higher latex content, the composite stiffness tends to be more dependent on the stiffness of the pure latex. The AA-SSA dispersant creates strong pigment–latex interfacial adhesion in dry composites, which is reflected in high elastic modulus and tensile strength. The AA-AMPS dispersant formulations had greater resistance to water. Due to the compatibility between the AMPS blocks and the SBR latex within the composite, higher storage modulus stability in water saturated composites is measured (at room temperature 56% of the storage modulus is preserved).     

粒度对高温色料色度的影响

运用离子呈色理论、晶体场理论,较好地解释了颜色釉的呈色机理,并根据色料的晶体结构及在釉中的存在状态,借助粒度分析仪、色度仪研究了色料的粒度大小对色度的影响规律.     

DEM simulations of the particle flow on a centrifugal fertilizer spreader

Usually, the performance of centrifugal spreaders must be evaluated in large halls by capturing the fertilizer distribution patterns in standardized tests, often carrying a big cost to the manufacturers. In contrast, this paper proposes a first attempt to model a particle flow subjected to a spinning disc using the Discrete Element Method (DEM) starting from the particle outflow of a bin, using flat as well as inclined discs. The model is validated by experiments in two different ways. The first manner is the measurement of the cylindrical mass distribution along the edge of the disc by a device that collects the fertilizer particles in a tray of baskets around the disc. A second method consists of collecting the particles on the ground after their ballistic flight through the air. Both validation methods are relatively cheap and fit into the present statistical or qualitative interpretation of DEM simulations. Additionally, a number of rotational disc speeds is chosen (300-650 rpm) to incorporate velocity dependent effects of the particle flow. It was found that the DEM simulations show a good qualitative and considerable quantitative agreement with the experiments. The deviations between the simulations and experiments are profound at high disc rotational speeds (500-650 rpm) and can be identified as (1) an underestimation of the simulated particle velocities at the edge of the disc and (2) a too low dispersion on the (vertical) simulated particle velocities at the edge of the disc. A parameter study revealed that (1) can be resolved by introducing a velocity dependent friction coefficient, in agreement with literature. The influence of other model parameters such as particle damping and stiffness appears to be small, while the introduction of a rolling friction coefficient to mimic rolling resistance or particle shape does not provide any answer either, and hence reason (2) at this moment must be addressed to unknown external factors such as disc plane vibrations appearing at higher disc speeds.     

DEM investigations of fluidized beds in the presence of liquid coating

In this work, the impact of liquid coating on fluidized bed behavior is studied by performing DEM investigations. A wet coefficient of restitution model, accounting for the viscous and capillary forces in addition to the inelasticity of particles, is implemented in an open-source numerical tool — MFIX-DEM. The modified numerical tool is used to study the effect of the coating thickness and viscosity on the operation of a bed consisting of mono-sized solid spherical particles pre-coated with the same film thickness. The simulation results show that as the coating viscosity is increased, particles tend to stay close together, hence the fluidization behavior changes and the air passes through the system in the form of slugs. It is also shown that as the coating viscosity is increased the time-average wet coefficient of restitution, normal relative collision velocities, and bed height decrease. The effect of increasing the coating thickness is similar to the above, i.e. it results in a reduction of the time-averaged wet coefficient of restitution and normal relative collision velocities.     

Discrete element simulation of SiC ceramic containing a single pre-existing flaw under uniaxial compression

A model of a SiC ceramic containing a single pre-existing flaw was established based on the discrete element method. The effects of the flaw inclination angles, which ranged from 0° to 75°, on the mechanical properties of the specimen under uniaxial compression were studied. The evolution of the force-chain field, displacement field and stress field around the pre-existing flaw in the process from the load to failure was also analysed. The results showed that the flaw inclination angle affected the mechanical properties of the specimen as well as the initiation and propagation of the first crack. Based on the investigation of the force chain field, it was found that the distribution curve of the normal force carried by the parallel bond in the specimen with the corresponding angles under compression is similar to the “peanut” rose diagram, while the shear force distribution curve is similar to the "butterfly wings" rose diagram. In addition, in the analysis of the displacement field and the stress field, the displacement field around the flaw can be divided into four types in the process from specimen loading to its failure. Meanwhile, it was found that initiation of the first crack was affected by tensile stress. With the propagation of the first crack, the tensile stress concentration region at the flaw tip moved and dissipated correspondingly.     

Simulation of the consolidation of paper coating structures: probabilistic versus deterministic models

Two categories of mathematical models were compared for the simulation of consolidation of paper coating structures, that is for the packing of pigments on a paper substrate under dewatering conditions. The first category uses probabilistic methods, relying on a random number generator to either determine the initial position of the pigments or their motion. The second category uses deterministic methods based on force balances. In this work, two probabilistic models and two deterministic models are described and their respective advantages and drawbacks are critically reviewed. Simulation results obtained using three of these methods are compared for the case of monodisperse and bidisperse spherical suspensions. Porosity calculations of the numerical packings obtained with the (deterministic) discrete element method (DEM) and two probabilistic methods, the Monte-Carlo (MCD) and the steepest descent (SDD) deposition methods, are compared with experimental data from the literature. These calculations reveal significant differences in the pore volume obtained with these three models. An analysis based on the bridging and relaxation phenomena that prevail in the flow of such particulate systems provide an explanation for these differences and show the strong potential of the discrete element method.

The choice of the simulation method depends on the objective of the simulations. DEM will provide more accurate predictions of macroscopic quantities such as the porosity or the roughness, but requires very long computational times. MCD or SDD will only provide qualitative trends, but is computationally far less intense. A combination of strategies might be appropriate, using MCD (or SDD) to provide guidelines and DEM to enhance the results predicted by MCD.     

High speed compression of highly filled thin composites: Effect of binder content and stiffness

A novel platen press has been utilized to study the effect of binder stiffness and content on the high speed compression behavior of the paper coating composites. Coatings consisting of four different styrene–butadiene latex binders with different glass transition temperatures were tested in this study. Also, the response of structures containing 5, 10, 12 and 15.5 pph of latex will be discussed. Furthermore, experimental results will be compared to simulation results obtained by discrete element models (DEMs), which are created based on the micro-properties of each component. DEM was capable of predicting both the trends and the quantitative changes in the responses of the structures under compressive force. It was also noted that as the density of the composite structure increased with increasing latex concentration, the stiffness of coating structures increases. This highlights the influence of voids on the stress transfer within the structures. The results of this study provide insight into the compression and smoothening phenomena of different paper coatings during calendering and fusing processes.     

Predicting discharge dynamics from a rectangular hopper using the discrete element method (DEM)

Accurate prediction of the discharge rate from hoppers is important in many industrial processes involving the handling of granular materials. The present work investigates the parameters affecting the discharge rate using the discrete element method (DEM). The effects of particle properties (particle size and size distribution) and hopper geometry (hopper width, outlet width, angle and fill height) are studied and compared to previously published experimental correlations. The results indicate that DEM simulations are fully capable of reproducing trends in the discharge rate that are well-known experimentally. For example, particle size and hopper width are shown to have a minimal influence on the discharge rate. In addition, for rectangular hoppers, the discharge rate is shown to vary with the outlet width raised to the power as given by the modified Beverloo correlation. The DEM simulations are also used to explore a wider range of parameters that have not been or are not easily explored experimentally. For example, the effects of hopper friction, particle friction, coefficient of restitution are investigated, and particle friction is shown to have a significant influence on the hopper discharge behavior.     

Influence of temperature on the structure of SiC coatings prepared by dynamic ion mixing

We deposited silicon carbide films, 0.5 μm and 0.86 μm thick at room temperature (RT) and 750 °C on (100) silicon wafers and TA6V substrates. An SiC target was sputtered with a 1.2 keV Ar⁺ ion beam delivered by a Kaufman-type ion source, and the growing films were continuously bombarded with a beam of 160 keV Ar⁺ ions. The microstructural state of the films was investigated by complementary techniques: transmission electron microscopy (TEM), high-resolution TEM, glancing X-ray diffraction (GXRD) and Fourier transform infrared spectroscopy (FTIR). All these characterization methods show that the bombardment of the growing films induces important structural changes. The SiC films prepared at RT without mixing are amorphous, whereas those deposited by dynamic ion mixing (DIM) at RT exhibit the beginning of crystallization of the β-SiC phase. At 750 °C the films prepared by DIM are formed of nanocrystallized grains of the cubic β-SiC phase.     

The effect of pore morphology and agarose coating on mechanical properties of tricalcium phosphate scaffolds

Three-dimensional biocompatible porous structures can be fabricated using different methods. However, the biological and mechanical behaviors of scaffolds are the center of focus in bone tissue engineering. In this study, tricalcium phosphate scaffolds with similar porosity contents but different pore morphologies were fabricated using two different techniques, namely, the replica method and the pore-forming agent method. The samples fabricated using the pore-forming agent showed more than two times higher compressive and bending strengths and more than three times higher compressive moduli. Furthermore, a thin layer of agarose coating improved the compressive and bending strength of both types of ceramic scaffolds. Subsequently, the samples’ capability to guide biomineralization was evaluated by immersion into a simulated body fluid that developed Ca-P nano-platelets formation and enhanced the compressive strength. Finally, the tetrazolium-based colorimetric (MTT) assay was used to evaluate L929 cell viability and proliferation on all the samples and confirmed that cell behavior was not affected by pore morphology or agarose coating. In summary, samples produced by the use of the pore-forming agent showed higher potential to be applied as bone scaffolds in tissue engineering applications.     

The effect of chemically bonded organic surface layers on the behaviour of fine powders

We have examined the effects of chemically bonded organic surface layers on the behaviour of a ceramic and a metal powder. The thin monomolecular layers were found to improve the corrosion resistance of the sensitive Nd-Fe-B powder and make the fine alumina powder hydrophobic and, as a result, making both coated powders significantly less sensitive to environmental humidity. Our analysis of the flowability of the noncoated and coated powders revealed that the alumina powder can be classified as very cohesive—the addition of the coating even increased its cohesivity. The Nd-Fe-B powder was determined to be easy flowing, and the effect of the coating changed with the applied normal stress. The coating does, however, improve the powder-compaction properties of both powders. In addition, the beneficial effect of the coating in terms of viscosity as well as in resistance to destabilization was also observed for suspensions in organic solvents.     

颜填料体积浓度的用量对涂料保色性的研究

阐述了配色原理,分析了不同PVC用量对涂膜的附着力、硬度、耐水、耐酸等性能的影响,得到PVC的用量在48%时,涂料的各种性能达到最佳,同时还给出了用来配深、中、浅各种颜色的白色涂料PVC的范围及相应的色浆用量,采用控制颜填料体积浓度的用量,解决了涂料保色性的问题;通过扫描电镜分析、粒度分析对比,观察了涂料的外貌和平均粒径,结果表明,在PVC为48%时,涂料的分散效果及稳定性均好。     

Study of the effect of liquid bridges on the dynamic behavior of two colliding tablets using DEM

By incorporating a viscous liquid force model into DEM simulations, the effect of liquid bridging between two tablets during a collision was studied. It was shown that when compared to the viscous force, the capillary force is several orders of magnitude smaller. In order to predict the capture of a falling tablet by the liquid bridge, it was necessary to adjust the limiting separation distance used in the viscous force model. Comparison of experimental results with the simulations, using oils with viscosities in the range of 100-1,000 cP, showed that there was a linear relationship between the limiting separation distance and the viscosity of the silicone oils. This viscosity-dependent separation distance varied in the range of 1-6 mm.     

Numerical simulations of diametrical compression tests on agglomerates

The paper reports discrete element simulations of the diametrical compression test applied to two spherical agglomerates: one a dense agglomerate and the other a loosely packed agglomerate. The results obtained for the dense agglomerate show that the agglomerate fractures along a slightly inclined, approximately diametrical plane. Outwardly, the agglomerate shows all the characteristics of brittle fracture but half of the final number of broken bonds was progressively broken during loading. In the simulation on the loose agglomerate, significant flattening occurred at the platens and the agglomerate failed by crushing.     

A study on tangential force laws applicable to the discrete element method (DEM) for materials with viscoelastic or plastic behavior

The discrete element method is a widely used particle orientated simulation approach for modeling granular systems. It is based on tracking each particle's movement and its interactions with the surroundings over time. The motion of a particle is given by a system of coupled ordinary differential equations which are solved numerically. Therefore, models for the forces acting between particles in contact need to be specified. In the past, detailed investigations dealing with the accuracy of tangential force-displacement models have been very limited, with sparse experimental data considered and the frequent restriction of including only fully elastic materials. In large scale discrete element simulations, on the other hand, viscoelastic or plastic material behavior is often assumed for normal contacts and combined with arbitrary tangential models. To address this situation a number of tangential force-displacement models are reviewed including linear models by Cundall and Strack [1979. A discrete numerical model for granular assemblies, Geotechnique 29, 47-65], Di Maio and Di Renzo [2004. Analytical solution for the problem of frictional-elastic collisions of spherical particles using the linear model. Chemical Engineering Science 59(16), 3461-3475], Brendel and Dippel [1998. Lasting contacts in molecular dynamics simulations. In: Herrmann, H.J., Hovi, J.-P., Luding, S. (Eds.), Physics of Dry Granular Media, Dordrecht. Kluwer Academic Publishers, pp. 313], Walton and Braun [1986. Viscosity, granular temperature and stress calculations for shearing assemblies of inelastic, frictional disks. Journal of Rheology 30, 949] and simple non-linear models by Brilliantov et al. [1996. Model for collisions in granular gases. Physical Review E 53(5), 5382-5392], Tsuji et al. [1992. Lagrangian numerical simulation of plug flow of cohesionless particles in a horizontal pipe. Powder Technology 71, 239-250] and Di Renzo and Di Maio [2005. An improved integral non-linear model for the contact of particles in distinct element simulations. Chemical Engineering Science 60(5), 1303-1312]. Whereas for fully elastic materials the parameters of the tangential force-displacement models can be derived directly from mechanical properties a scaling approach is proposed for the estimation of the parameters in the non-elastic case. The effect of different normal force-displacement models is analyzed. For all model combinations macroscopic final collision properties are derived and compared to experimental results by Foerster et al. [1994. Measurements of the collision properties of small spheres. Physics of Fluids 6(3), 1108-1115], Lorenz et al. [1997. Measurements of impact properties of small, nearly spherical particles. Experimental Mechanics 37(3), 292-298], Gorham and Kharaz [2000. The measurement of particle rebound characteristics. Powder Technology 112(3), 193-202] and Dong and Moys [2003. Measurement of impact behaviour between balls and walls in grinding mills. Minerals Engineering 16(6), 543-550; 2006. Experimental study of oblique impacts with initial spin. Powder Technology 161(1), 22-31].